1. Field of the Invention
The invention relates to a method for controlling attitudes of a satellites in elliptic orbits or satellites initially placed in circular orbits perturbed to elliptic orbits due to environmental disturbances.
2. Background of the Related Art
Hereafter, a satellite means any artificial object in the solar system in orbit around the Earth or any other planet or object of the solar system, or, in solar orbit. The attitude stability of satellites represents aspects of considerable importance for successful completion of a space mission. Unfortunately, even though a satellite may be precisely oriented at the beginning, it deviates in time from its preferred orientation under the influence of environmental forces like gravity gradient, solar radiation pressure, magnetic, aerodynamic, free molecular reaction forces, and others. However, these forces, if properly utilized, may stabilize the attitude of the satellite instead of deteriorating it. The solar radiation pressure (SRP) for attitude control of high altitude satellites and interplanetary probes has been proposed in several patents and journal papers.
French Patent No. 2,513,589 discusses a method and a device for aligning the roll axis of a spin-stabilized satellite along a required direction. The satellite is fitted with fixed solar panels and mobile surfaces mounted at the ends of the panels.
French Patent No. 2,550,757 proposes the position control of the satellite by deforming the solar panels to impose a variable backwards curvature on each of them.
French Patent No. 2,529,166 discloses a satellite station-keeping method using solar sails and a space vehicle implementing this method.
German Patent No. 2,537,577 explains the provision at the end of the solar panel surfaces that can be oriented about the axis of the solar panels and transversely thereto.
U.S. Pat. No. 3,304,028: “Attitude Control for Spacecraft”, is similar to French Patent No. 2,513,589, previously mentioned, as is U.S. Pat. No. 3,339,863.
French Patent No. 2,530,046 entitled “Geosynchronous Satellite Attitude Control System” proposes the variation of the relative orientation of the solar panels about their axes similar to U.S. Pat. No. No. 4,325,124 entitled “System for Controlling the Direction of the Momentum Vector of a Geosynchronous Satellite”.
European Patent No. 0,295,978 presents a device and a method for pointing a space probe towards a heavenly body. North and South solar sails are added to the satellite having asymmetrical surface areas, orientations about a North-South axis or inclinations transverse to this axis.
French Patent No. 2,552,614 proposes a satellite configuration with improved solar means comprising solar panels oriented transversely to the North-South axis and adapted to be oriented about axes transverse to the North-South axis.
U.S. Pat. No. 4,262,867 explains the position and attitude control of a spacecraft by solar panels and vanes adapted to be partially retracted accordion fashion relative to the direction of the ambient SRP.
U.S. Pat. No. 4,591,116 proposes the attitude control of a spacecraft accomplished by changing the effective surface area of the solar panels facing the sun in accordance with certain rules of asymmetry and utilization of pulling cables.
U.S. Pat. No. 4,747,567 describes the attitude control of a spacecraft by an articulation device for changing the position of the solar array relative to the spacecraft body independently about all three axes.
U.S. Pat. No. 5,310,144 explains a method and apparatus for balancing SRP and gravity-gradient disturbance torques acting upon a satellite are based upon tilting of the satellite solar panels toward or away from the sun with respect to the satellite body-fixed frame.
U.S. Pat. No. 5,850,992 provides a method for controlling the pitch attitude of a geostationary satellite is suggested by tilting the satellite solar panels using an on-board computer. The tilting is also used to position the center of gravity of the satellite during orbit control maneuvers so as to minimize the disturbances caused by an offset between the center of gravity and the real thrust vector of orbit control thrusters.
Various configurations such as a trailing cone system, weathervane type tail surfaces, a reflector-collector system, corner mirror arrays, solar paddles, grated solar sails, and mirror-like surfaces have been suggested for properly utilizing SRP torque. These concepts have been applied to sun-pointing satellites and gravity-oriented satellites. Spinning as well as non-spinning satellites was also considered. The attitude control of the satellite has been accomplished by translational motions of single or several control surfaces relative to the satellite body or by rotating the control surfaces about satellite body-fixed axes. Some missions have also been flown to verify these concepts involving SRP for the satellite attitude control. The Mariner IV mission employed solar vanes for achieving passive sun pointing attitude. The European Space Agency conducted experiments in which the attitude of the geostationary communication satellite OTS-2 was controlled by rotating solar panels.
Thus, the SRP control torque can be utilized to stabilize vibrational dynamics of a satellite with a desired degree of accuracy. However, in the case when the satellites are in elliptic orbits, the amplitude of attitude oscillations increases rapidly with increase in eccentricity. Even though there are no attitude disturbances initially, the attitude motion gets excited and become unstable in the presence of high eccentricity. To overcome the adverse effect of eccentricity, Kumar and Joshi applied the SRP torque to control the attitude of axis-symmetric satellites by regulating the translatory motions of the control surfaces relative to the satellite body. However, instead of applying translatory motions of the control surfaces, it would be better and easy if we can stabilize the satellite by the rotating control surfaces and that too if the angle of rotation is small on the order of a degree or fraction of a degree. Such investigations concerning the attitude control of satellites in elliptic orbits by rotating the control surfaces have not yet been discussed in the literature. In the present invention, we provide a method for resolving this problem for an earth-oriented non-axis-symmetric satellite.